Re: [PATCH 43/49] drm/i915/bdw: Handle context switch events

[Damien Lespiau <damien.lespiau@intel.com>]

On Thu, Mar 27, 2014 at 06:00:12PM +0000, oscar.mateo@intel.com wrote:
> +void gen8_handle_context_events(struct intel_engine *ring)
> +{
> +	struct drm_i915_private *dev_priv = ring->dev->dev_private;
> +	u32 status_pointer;
> +	u8 read_pointer;
> +	u8 write_pointer;
> +	u32 status;
> +	u32 status_id;
> +	u32 submit_contexts = 0;
> +
> +	status_pointer = I915_READ(RING_CONTEXT_STATUS_PTR(ring));
> +
> +	read_pointer = ring->next_context_status_buffer;
> +	write_pointer = status_pointer & 0x07;
> +	if (read_pointer > write_pointer)
> +		write_pointer += 6;
> +
> +	spin_lock(&ring->execlist_lock);
> +
> +	while (read_pointer < write_pointer) {
> +		read_pointer++;
> +		status = I915_READ(RING_CONTEXT_STATUS_BUF(ring) +
> +				(read_pointer % 6) * 8);
> +		status_id = I915_READ(RING_CONTEXT_STATUS_BUF(ring) +
> +				(read_pointer % 6) * 8 + 4);
> +
> +		if (status & GEN8_CTX_STATUS_ELEMENT_SWITCH) {
> +			if (check_remove_request(ring, status_id))
> +				submit_contexts++;
> +		} else if (status & GEN8_CTX_STATUS_COMPLETE) {
> +			if (check_remove_request(ring, status_id))
> +				submit_contexts++;
> +		}
> +	}
> +
> +	if (submit_contexts != 0)
> +		gen8_switch_context_unqueue(ring);
> +
> +	spin_unlock(&ring->execlist_lock);
> +
> +	WARN(submit_contexts > 2, "More than two context complete events?\n");
> +	ring->next_context_status_buffer = write_pointer % 6;
> +}

I'm a bit suprised that we never update the read pointer in the
CONTEXT_STATUS_PTR when we consume entries from CONTEXT_STATUS_BUF.

Are we sure this field isn't used by hw at all to figure out if the
circular buffer has some free space?

-- 
Damien